A form of Names to Run Away from Really Fast: Some people end up going by a pseudonym where they have a one-word description followed by an X. It's generally not a good idea to press them for real names.
Devoted members of the Audubon Ballroom from Honor Harrington use "X" as their only last name; special mention goes to Jeremy X. Definitely run from the Ballroom if you're associated with the slave trade.
CSI: New York had a storyline about a female Professional Killer who they only referred to as "Suspect X." Her crimes included killing an Internet celebrity to take her identity, assassinating a federal judge, holding hostage, surgically altering and finally killing a random innocent woman to fake her own death, and hacking Second Life.
Red X; not just the only antihero in the series but also the only character to never be defeated! (from the animated Teen Titans, with a color to boot!)
Beast Wars gives us Protoform X, complete with a big ugly X on his stasis pod. Prior to getting sealed in that stasis pod, he was on a massive spree of murder, destruction, and cannibalism, with a special focus on destroying everything Depth Charge ever cared about. Becomes the aptly named Predacon Rampage once he's released.
Malcolm X, why not. Real life Badass Preacher extraordinaire, not to mention (at least from his own perspective) an epic Heel–Face Turn when he left the black separatist Nation of Islam for mainstream Islam.
There is an as of yet unidentified pliosaur known as Predator X. Estimates put its length at about 50 feet and give it a bite force more powerful then a Tyrannosaurus rex.